1. Field of the Invention
The present invention relates to a raindrop sensor, and more particularly to a raindrop sensor, which is preferably applied to, for example, a wiper controller for a vehicle.
2. Description of Related Art
Japanese Unexamined Patent Publication No. 2001-66246 corresponding to U.S. Pat. No. 6,507,015 discloses a raindrop sensor, which is mounted on an inner surface of a windshield and senses raindrops attached to an outer surface of the windshield. The raindrop sensor includes a light emitting element, a light guide body, and a light receiving element. The light emitting element emits light in a direction from the inner surface to the outer surface. The light guide body guides the light to the outer surface, and also guides a reflected light, which is reflected by the outer surface, toward the inner surface side. The light receiving element receives the light from the light guide body, and generates a signal according to an amount of the received light. The raindrop sensor compares an amount of the light received in a clear whether condition with a current amount of the light currently received in order to determine whether moisture, such as raindrops, is attached to the outer surface or not.
Also, Japanese Unexamined Patent Publication No. 2001-521158T corresponding to U.S. Pat. No. 5,898,183 discloses a raindrop sensor, which includes a light guide body having a prism body and a flexible interlayer. In the raindrop sensor, a silicone sheet, for example, serves as the interlayer, which is held between the windshield and the prism body. Thus, this limits an air layer from forming between the prism body and the windshield such that the light, which travels through the prism body, can be guided to the outer surface of the windshield.
However, in assembly of the raindrop sensor to the inner surface, when the prism body is attached to the inner surface directly without the interlayer, the air layer is formed between the prism body and the inner surface. Then, the light, which travels through the prism body, may not reach the windshield, but may reflects off the end face of the prism body. Thus, the raindrop may not be detected because the light emitted by the light emitting element does not sufficiently reach the outer surface of the windshield.
To deal with the above disadvantages, in a factory for attaching the raindrop sensor, an operator checks whether the silicone sheet, which is a part of the light guide body, is appropriately attached or not by visual examination. However, because this silicone sheet is provided at the back of the raindrop sensor and thus the silicone sheet is behind a cover thereof, it is often difficult for the operator to check by the visual examination.